FEL Pulse Temporal Profile Made in a FLASH

The temporal profile of an individual free-electron laser (FEL) pulse now can be measured with femtosecond precision using FLASH, a soft-x-ray FEL. The technique could be used to film atoms in motion or to study chemical reactions and phase transitions.

Ultrashort and ultra-intense x-ray pulses delivered by FELs provide unique research opportunities. Trillions of x-ray photons are packed within a single burst, which lasts for only several tens of femtoseconds, or even less. However, the precise arrival time and the temporal profile of the pulses can change dramatically from one pulse to the next. For ultrafast dynamic processes to be filmed, the arrival time of each pulse must be measured to reorder the individual frames or snapshots captured with individual FEL pulses.

Now, by using the FLASH at DESY, Germany’s particle accelerator, an international team of scientists has developed a measurement technique that provides a complete temporal profile of an individual FEL pulse. The method uses an adapted attosecond science technique called photoelectron streaking, which “permits recording temporal profiles of varying light signals by creating photoelectron bursts and measuring the energy distribution of these electrons,” said research professor Andrey Kazansky of Donostia International Physics Center and the University of The Basque Country. The method can be implemented at any of the world’s x-ray FELs.

An electromagnetic field accelerates photoelectrons emitted from neon atoms irradiated by an x-ray free-electron laser. In this way, the x-ray pulse temporal profile and arrival time are uniquely retrieved on a pulse-to-pulse basis with femtosecond precision. Courtesy of Jörg Harms/MPSD at CFEL.

Taking advantage of the FEL’s ultrahigh intensities, the researchers performed the streaking measurement on a single-shot basis. In the experiment, x-ray flashes were shot through neon gas on their way to the target. Each x-ray pulse ejects a burst of photoelectrons — the electron emitted from matter as a consequence of the absorption of a high-energy photon — from the noble gas. It turns out that the photoelectron burst’s temporal profile is a replica of the FEL pulse that ejected them.

A very intense electromagnetic field is then used to accelerate or decelerate the photoelectrons, depending on the exact instant of their ejection. The strength of this effect is measured. When all of the information is combined, the temporal profile and arrival time of the individual x-ray pulses can be obtained with a precision of about 5 fs.

“Simultaneous measurement of the arrival time and pulse profile, independent of all other FEL parameters, is the key to this technique,” said Adrian Cavalieri, a University of Hamburg professor and a group leader in the Max Planck Research Dept. for Structural Dynamics. Until now, no other measurement has provided this complete timing information.

With simultaneous measurement of the FEL x-ray pulse profile, it will be possible to explore processes that evolve within the x-ray exposure. On these timescales, the motion of electrons and electronic state dynamics become significant. Electronic dynamics drive damage processes in biomolecules, which may destroy them before they can be recorded in a crystal clear image.

Because the measurements are made without affecting the FEL beam, they can be applied to any experiment at almost any wavelength.

The findings will be used to monitor and maintain the FEL pulse duration at FLASH to study a variety of atomic, molecular and solid-state systems. Down the road, the scientists plan to use these high precision measurements as critical feedback for tailoring and manipulating the x-ray pulse profile.

Scientists from the Center for Free-Electron Laser Science in Hamburg and Lomonosov State University in Moscow also contributed to the study, which appears in Nature Photonics (doi: 10.1038/nphoton.2012.276).

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